PdeKeyListener.java: tab key behaves correctly if !editor.tabs.expand

If the `editor.tabs.expand` preference setting is set to `false` (tabs 
are *not* expanded to spaces), insert a tab character when the tab key 
is pressed. Previously nothing would be inserted.

Current (incorrect) behaviour:
![processing-expand-tab-2014 06 22](https://cloud.githubusercontent.com/assets/342964/3352193/470ef188-fa37-11e3-8ffd-81f5cce20844.gif)